The invention relates to a method of transferring data from a transponder to a reading device in a scanning identification system by means of load modulation of an inquiry signal emitted by the reading device.
In the last few years, a system for the identification of persons, animals and objects that has proven effective involves the supply of a transponder, which is connected to the object to be identified, with energy by a (stationary or portable) reading device, also referred to as a reader or base station, via an alternating field, whereupon the transponder responds by emitting the data it stores. Because of the frequency range used, the system is referred to as a radio-frequency identification system, or RFID for short.
An RFID transponder generally comprises an antenna coil and an integrated circuit containing all of the necessary electronic circuit blocks, such as those needed for voltage supply, timing generation, timing control and storing the data required for the identification. The capacity switched in parallel to the antenna coil is likewise often a component of the integrated circuit. It can, however, also be formed by a discrete component.
The RFID reading device comprises a resonating circuit having a transmitting coil and a capacitor that is actuated by a driver stage with a signal generally having a fixed frequency (e.g., 125 kHz). The reading device further includes electronic circuit blocks for recognizing data emitted by the transponder through absorption modulation, and for transmitting data and commands, e.g., through field modulation, to the transponder.
During the data or energy transfer, the reading device and the transponder form a loosely-coupled transformer. The energy transfer is therefore relatively low.
In the majority of identification applications of transponder/reading device systems, it is desirable and advantageous for the identification, that is, the exchange of data between the transponder and the reading device, to be effected without a delay. For example, a problem in the use of an anti-theft device is that, when a motor vehicle is started, the driver notices a time delay of more than 150 ms from the turning of the ignition key until the engine starts. Consequently, the entire transfer protocol must have run during this very short time period.
When the field is switched on, the reading device requires approximately two milliseconds before the high coil voltage of about 100 V has built up and the useful signal at a level of a few millivolts, which results from the modulation of the transponder, can be evaluated, and the data can thus be decoded correctly. The first ID code transmitted by the transponder is then recognized in the second sequence, which means a time delay of the data protocol. A similar problem occurs during the transition from a non-damped to the damped state, for example at the end of a writing sequence or programming sequence.
It is therefore the object of the invention to provide a method of transferring data from a transponder to a reading device in a scanning identification system through load modulation of an inquiry signal emitted by the reading device, the method providing the correct result through a one-time data transfer.
The above object generally is accomplished according to the invention. More specifically, in a method of transferring data from a transponder to a reading device in a scanning identification system through load modulation of an inquiry signal emitted by the reading device, with the transponder being initialized at the beginning of the reception of the inquiry signal and responding with the emission of the response signal, first a predetermined number of data bits that are not part of the response signal is transmitted, and a terminator sequence followed by the response signal is then repeatedly transmitted, until the inquiry signal is interrupted.
The predetermined number of data bits that are not part of the response signal bridges the time span required by the reading device for building up the inquiry field. At the same time, the reading device is synchronized to the clock of the response signal. With the terminator sequence, which indicates the beginning of a valid response signal, the response signal is already recognized correctly by the reading device during the first transfer.
The terminator sequence advantageously comprises a bit sequence that cannot occur in the actual response signal. For example, in the transmission of the terminator sequence, the inquiry signal is unmodulated for 1.5 bit periods. A sequence of this type cannot occur in the actual response signal, and is therefore reliably recognized by the reading device. It is also possible to load-modulate the inquiry signal for 1.5 bit periods during the transmission of the terminator sequence. The time frame of 1.5 bit periods is crucial for reliable recognition as a terminator sequence by the reading device.
A sequence of three to ten logical ones is transmitted as a predetermined number of data bits not belonging to the response signal. Within this time, the inquiry signal has generally built up to the point where the reading device can reliably decode the load-modulated response signal.